1. Field of Invention
This invention relates to flexible panels or sheet structures having internal evacuated cavities, specifically to panels of this type which are used to provide vacuum thermal insulation.
2. Description of Prior Art
The thermal insulating effects of a vacuum are well known. When the pressure of a gas is reduced to a high vacuum its ability to transmit heat by conduction or convection becomes substantially zero. Here a high vacuum is defined as one having a pressure less than 0.1 Newton per square meter (approximately one millionth of normal atmospheric pressure). Unlike conventional, gas-filled, thermal insulating layers, the insulating effectiveness of a vacuum is not a function of the thickness of the evacuated cavity. That is, the evacuated cavity may be very thin while still providing highly effective thermal insulation. Evacuated cavities are also useful in insulating high electric voltages. Another use for evacuated cavities is as acoustic insulators.
Many structures have been disclosed that use the insulating effect of a vacuum. Evacuated insulation requires a sealed, double-walled container defining an internal cavity held at a high vacuum. The structure must be resistant to the surrounding, ambient, pressure That ambient pressure averages 101,320 Newtons per square meter (14.7 pounds per square inch) in air at sea level.
There are many applications for an insulating structure that is thin in comparison to its width and length, i.e. a panel or sheet-like structure. All prior-art evacuated panel structures have two external sheets, joined at their edges to form an internal space, with internal members that act as spacers between the external sheets. Such evacuated insulating panels must satisfy three principal requirements: resistance to collapse, low-conductivity internal spacers, and impermeability to external gas flow into the interior evacuated space.
The forces acting to collapse the evacuated panel act in two ways. The ambient pressure first acts directly against the large area of the external sheets to force them together. Here that is called a facewise force. A second collapsing action is due to that portion of the ambient pressure that acts on the edges of the evacuated panel, causing them to be forced toward the middle of the panel. Here that is called an edgewise force.
The internal spacers resist the compressive stresses produced by the facewise force. Nearly all heat conduction through the middle of evacuated panels is through the internal spacers. To limit that thermal conduction the internal spacers are usually made of materials having low thermal conductivity. An alternative method to limit thermal conduction uses spacers designed with at least one portion having a limited cross-sectional area or increased length.
The external sheets of evacuated panels must have at least one impermeable layer. Without an impermeable layer, passage of gases from the exterior would eliminate the internal high vacuum and cause the insulation to become ineffective. According to "Ultrahigh Vacuum Practice" by George Frederick Weston, Butterworth & Co. Ltd. 1985 (ISBN 0-408-01485-7), the quantity of gas passing through a solid wall is proportional to a "permeation constant" and inversely proportional to the thickness of the wall. The permeation constant varies with temperature, the particular gas, and the material of the wall. Glasses and ceramics are generally impermeable, but are also rigid and brittle. Polymers (plastics) and rubbers are flexible, but are generally orders of magnitude more permeable to gases than glasses and ceramics. Only metals combine good impermeability with an ability to flex in use to a useful degree.
All prior evacuated insulating panels have been rigid, or only semi-flexible. This is the result of attempts by prior designers to satisfy the three principal requirements: resistance to collapse, low-conductivity internal spacers, and impermeability. In U.S. Pat. No. 4,468,423 (1984) Hall discloses rigid parallel external sheets for the panel walls, to resist collapsing forces. In U.S. Pat. No. 4,420,922 (1983) Wilson discloses internal ribs as spacers, allowing thinner material to be used in the external sheets, but the spacer ribs act to stiffen the walls. Prior evacuated insulating panels have generally used a single layer of material in the external sheet to provide both resistance to collapse and impermeability. In U.S. Pat. No. 3,936,553 (1976) Rowe discloses impervious surface sheets of generally rigid materials. Rowe's use of sheets made of highly-impermeable aluminum would allow the external sheets to be relatively thin, but these sheets would be subject to cracking and failure if flexed in use. Both Rowe and Wilson show pre-shaped external sheets to provide even more rigidity than provided by a simple flat sheet design.
Taylor, in U.S. Pat. No. 4,791,773 (1988), discloses thin walls with a separate plastic sheet to act as an impermeable layer. Plastic in thin layers is flexible, but as noted earlier, is highly permeable to gases, especially in thin layers. Taylor also uses undulated spacers that act to stiffen the entire panel into rigidity.
Karpinski, in U.S. Pat. No. 4,304,824 (1981), describes a modular insulation quilt having flexible outer layers, with fused edges, surrounding a plurality of shaped foamed material in vacuum. The foamed material is formed into expanded pellets that act as spacers filling most of the interior. The expanded pellets, while of low conductivity, still provide heat paths through the middle of the panel and must have a substantial thickness to limit heat losses.
Karpinski's panel is not flexible. The pellet spacers are forced together strongly by atmospheric pressure forming a semirigid solid that would be difficult to bend by itself. The pellets are covered by an outer layer of hard nylon polyester or aluminum foil, which materials cannot stretch to any large degree before failure. The resulting panel is strongly constrained against flexure because the pellets on the concave side of a bend must be compressed further. Further, Karpinski's panel can conform to such a bend only with attendant buckling in the outer layer on the concave side of the bend. Repeated bending of Karpinski's panel would crack the outer layer at the buckling point and cause failure from loss of the vacuum.
Rathmell, in U.S. Pat. No. 4,317,854 (1982), describes a panel structure having two metal sides with corrugations three inches (80 mm) apart and 11/8 (26 mm) inches deep. He states that the corrugations can resist pressure up to 14.7 psi (10 Newtons per square centimeter) without bending. Adjacent corrugations from respectively opposite metal sides are joined in the interior by a web of glass fibers which are stressed in tension and are generally parallel to the panel. Rathmell states that such a panel may be curved to provide a cylindrical tank, implying a capability of limited flexure about an axis that is parallel to the corrugations.
Rathmell's panel is not flexible. This is indicated by his statement that such a panel would need to be specially "constructed to maintain an evacuated space around a 90.degree. corner," illustrating a limitation in the panel's inherent ability to flex. When bent, one corrugated sheet takes on an overall convex shape when seen from the outside, requiring the corrugation depth to decrease. The opposite sheet takes on an overall concave shape when seen from the inside, requiring its corrugation depth to increase. The sharpness of the bend is limited by the geometric limitation in the changes in corrugation depth before the two sheets touch, reducing the adaptability of Rathmell's panel. Second, his panel can only flex in one axis, parallel to the corrugations, and therefore could not be shaped to follow compound curves. The corrugations described by Rathmell would stiffen such a panel against bending on an axis orthogonal to those corrugations. Third, the edge seals shown by Rathmell show no special ability to lengthen or compress in such a way as to allow any substantial bending in the panel without failure. Fourth, his panel uses a monolithic wall structure of metal that supports collapsing forces and also forms the impermeable layer. To support the collapsing forces, the walls described by Rathmell must be thick, perhaps 1/16 to 1/8 inch (1.5 to 3 mm). With such thick walls, bending the sheet sets up forces in each sheet where one side is in tension and the other in compression. These differential forces in the sheet tend to crack the wall, making it quite vulnerable to cracks through fatigue failure. Upon cracking, the evacuated space would soon fill with gas from the exterior.
There is a particular limitation to rigid, or semi-flexible, evacuated insulating panels compared to rigid air-filled insulation. This is because it is not possible to cut or shape evacuated insulators to size without violating the integrity of the vacuum space. Therefore, rigid or semi-flexible evacuated insulating panels require that each panel be customized to fit each application, since the panels have only limited ability to change shape after fabrication. Therefore, mass production techniques cannot be applied to reduce the cost of making and applying rigid evacuated insulating panels.
In summary, prior evacuated panels or sheet insulators have the following disadvantages:
(a) Evacuated panels have been rigid, or only semi-flexible. They have no, or only a limited, ability to be sharply bent after fabrication without failure of the vacuum insulation. PA1 (b) Previous semi-flexible evacuated panels have only been bendable in one axis, and therefore could not be bent to follow a compound curve. PA1 (c) Previous semi-flexible evacuated panels have combined the structural elements necessary to resist collapsing forces and provide impermeability into a single monolithic material. Such a combination requires a thick layer of material, limiting the number of flexure cycles in such panels before fatigue failure causes cracks in the impermeable layer. PA1 (d) Previous rigid and semi-flexible evacuated panels require custom design and manufacture for most applications, precluding the benefits of mass production. PA1 (a) To provide an improved evacuated insulating panel that is highly flexible and can be bent into sharp curves after fabrication without failure. PA1 (b) To provide an improved evacuated insulating panel that can be bent in both planar axes, and therefore can be bent to follow a compound curve. PA1 (c) To provide an improved evacuated insulating panel that separates the structural elements, thereby providing resistance to collapse of the structural elements which provide impermeability. With such a separation, the panel's ability to withstand multiple cycles of bending without fatigue failure of the impermeable layer is greatly increased. PA1 (d) To provide an improved evacuated insulating panel that is useful in many end applications without the need for custom design and fabrication, due to its ability to be bent into a complex shape. This allows economies in its manufacture by mass production techniques.